1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, though not exclusively, a zoom lens that can be used in a still camera, a video camera, and a digital still camera and an image pickup apparatus having the zoom lens.
2. Description of the Related Art
In recent years, the market has desired image pickup apparatuses (e.g., cameras), such as a video camera and a digital still camera that use a solid-state image sensor, that have advanced functions and have thin body thicknesses. In addition, the market has desired that an optical system (e.g., a shooting optical system) that is used in the camera be a small zoom lens that has a reduced number of lens elements and has a high optical performance.
In such a type of camera, various types of optical members such as a low-pass filter and a color correction filter are disposed between a rearmost portion of the lens and the image sensor. Accordingly, it is desired that a zoom lens that is used for the above-mentioned camera has a relatively long back focal distance.
Conventionally, there have been proposed various short zoom type two-unit zoom lenses having a wide angle of field that include a first lens unit having a negative refractive power and a second lens unit having a positive refractive power and are configured to perform zooming by changing a lens interval between the lens units.
In the two-unit zoom lens, the second lens unit, having a positive refractive power, is moved to perform variation of magnification and the first lens unit, having a negative refractive power, is moved to compensate for variation of an image point position caused by the variation of magnification. Most two-unit zoom lenses have a zoom magnification (zoom ratio) of about 2.
A small zoom lens having a zoom ratio of 2 or higher includes a three-unit zoom lens in which a third lens unit, having a negative or positive refractive power, is located on an image side of the two-unit zoom lens. Japanese Patent Application Laid-Open No. 61-267721 and Japanese Patent Application Laid-Open No. 59-18917 discuss a three-unit zoom lens in which various kinds of aberrations occurring due to high zoom magnification are corrected by the third lens unit.
In addition, Japanese Patent Application Laid-Open No. 63-135913 and Japanese Patent Application Laid-Open No. 7-261083 discuss a three-unit zoom lens having a long back focal distance and a wide angle of field while securing good telecentric characteristics.
In addition, Japanese Patent Application Laid-Open No. 3-288113 discusses a three-unit zoom lens in which zooming is performed by moving a second lens unit, having a positive refractive power, and a third lens unit, having a positive refractive power, while maintaining a first lens unit, having a negative refractive power, stationary.
In addition, Japanese Patent Application Laid-Open No. 2004-94283, Japanese Patent Application Laid-Open No. 2004-239974, Japanese Patent Application Laid-Open No. 2004-318104, and Japanese Patent Application Laid-Open No. 2005-55496 discuss a small three-unit zoom lens in which a first lens unit includes two lens elements.
In addition, a conventional zoom lens used for an image pickup apparatus with a solid-state image sensor includes a small three-unit zoom lens in which various kinds of aberrations are corrected with an aspheric surface applied to a first lens unit so as to reduce the number of constituent lens elements.
Meanwhile, some conventional image pickup apparatuses are configured to electrically correct distortion among various kinds of aberrations using image processing instead of optically correcting distortion.
A zoom lens for a single-lens reflex camera that is designed for 35 mm film has a too long back focal distance to be applied to an optical apparatus (camera) that uses a solid-state image sensor. In addition, the zoom lens of this kind does not have good telecentric characteristics. Accordingly, if a zoom lens, for a single-lens reflex camera that is designed for 35 mm film, is directly applied to an optical apparatus that uses a solid-state image sensor, a phenomenon of shading occurs.
In recent years, attempts have been made to both implement the downsizing of a camera and increase the zoom magnification of a zoom lens that is used for the camera. The method for implementing the downsizing of a camera and increasing the zoom magnification includes a so-called lens retraction method. In the lens retraction method, the interval between lens units in a non-photographing state is reduced to an interval that is different from the interval in a photographing state so as to reduce the amount of protrusion of the lens from the camera body.
As the number of lens elements of each lens unit that constitutes a zoom lens is large, the length of each lens unit along an optical axis becomes large (that is, the whole length of the zoom lens becomes large). In addition, when the amount of movement of each lens unit during zooming or focusing is large, the whole length of the zoom lens becomes large. As a result, a desired length of the zoom lens with the lens units retracted cannot be obtained. Accordingly, it becomes difficult to utilize the lens retraction method. That is, as the zoom ratio of a zoom lens becomes higher, the whole length of the zoom lens becomes larger, and accordingly, it becomes difficult to apply the lens retraction method.
On the other hand, if one or more aspheric lenses are used in a zoom lens, the total number of constituent lens elements of the zoom lens can be reduced. Accordingly, the whole length of the zoom lens can be shortened. However, an aspheric lens is more difficult to manufacture than a spherical lens. In particular, an aspheric lens having a large effective diameter is more difficult to manufacture. Accordingly, increasing the number of aspheric lenses used in an optical apparatus or using an aspheric lens having a large effective diameter in an optical apparatus may cause a difficulty in manufacturing.
Accordingly, it is desirable that an aspheric lens is applied to a lens having as small an effective diameter as possible.
In general, it is relatively easy to realize a wide angle of view in a two-unit zoom lens that has a first lens unit, having a negative refractive power, and a second lens unit, having a positive refractive power, in order from an object side to an image side or in a three-unit zoom lens in which a lens unit having a positive or negative refractive power is disposed on the image side of the two-unit zoom lens. In addition, in this type of two-unit or three-unit zoom lens, a given back focal distance can readily be attained.
However, in configuring the whole lens system with a small number of lens elements while securing a good optical performance, it can be necessary to appropriately set the arrangement of refractive power of each lens unit and the shape of each lens element.
In particular, if the lens configuration of a first lens unit, whose effective diameter tends to be large, is set inappropriate, it is difficult to miniaturize the whole lens system while securing a high optical performance.